Sarajevo was besieged and bombarded from 1992, and by 1993 the blockade was total. Serb forces seized the airport, but handed it over to UN peacekeeping forces to avert international intervention. The UN used the airport to bring in humanitarian aid, but wouldn't allow any military support to the city's defenders, and they severed the city from Bosnian-controlled areas to the south. A secret tunnel was therefore dug between Dobrinja just north of the airport to Butmir just south: 340 m of true tunnel, plus covered trenches either side, totalling over 800 m. It enabled arms, munitions, oil and food to be brought into the city, and served as an evacuation route for officials and civilians. Later it was equipped with rail tracks, an oil pipe and electricity and telecoms cables. A museum has been built over the Butmir entrance, with 20 m of tunnel accessible to visitors, plus displays on the siege. City tours often include this museum.